Natural gas is combustible gas that is produced from underground gas fields and contains hydrocarbons as principle ingredients. However, in addition to hydrogen and light hydrocarbons such as methane, propane and butane, natural gas also contains sulfur compounds such as hydrogen sulfide and mercaptan and heavy hydrocarbons such as BTX (benzene, toluene and xylene) as well as carbon dioxide, oxygen and nitrogen. Particularly, sulfur compounds including hydrogen sulfide give off sulfur dioxide when burnt so that it is strongly desired to remove sulfur compounds from natural gas from the viewpoint of protection of the environment.
Sulfur compounds such as hydrogen sulfide and mercaptan and carbon dioxide that are contained in natural gas as impurities are generally referred to as acidic gas. It has been a general practice to remove such acidic gas in the course of refining natural gas according to the requirements in application thereof. More specifically, when natural gas is liquefied to LNG, the concentration of carbon dioxide, that of hydrogen sulfide and that of all the sulfur compounds put together are required to be not higher than 50 ppm (in volume ratio), not higher than 4 ppm (ditto) and not higher than 30 mg-S/Nm3, respectively. When natural gas is used as pipeline gas, the concentration of carbon dioxide need to be held to about 1% (in volume ratio) and sulfur compounds such as hydrogen sulfide and mercaptan have to be removed to a content level same as that of LNG.
Chemical absorption methods of absorbing acidic gas by means of an amine-based or carbonate-based alkaline aqueous solution are employed frequently as means for removing acidic gas from natural gas. When refining natural gas with this method, an amine process is popularly used because it is possible to optimize the refining system (in terms of the rate of circulation, the number of stages and the heat for regeneration) and reduce the cost. However, when mercaptan is contained in natural gas, it can hardly be removed by chemical absorption methods because mercaptan shows only a weak polarity and has only a weak bonding force relative to amine.
On the other hand, physical absorption methods of using an organic solvent prepared to absorb acidic gas components as a function of their respective partial pressures can absorb not only carbon dioxide and hydrogen sulfide but also other sulfur compounds such as mercaptan and disulfides. For example, the Sulfinol process of using a mixture of sulfolane and diisopropanolamine as absorbent solution is a combined type of a chemical absorption method and a physical absorption method that is being popularly used for absorbing and removing acidic gas from natural gas because it can efficiently remove hydrogen sulfide and mercaptan particularly when they are contained in natural gas to a large extent. However, since a physical absorption method generally absorbs not only acidic gas but also heavy hydrocarbons including BTX at the same time because of the nature of the absorbent solution it employs. Thus, when recovering sulfur from the separated acidic gas by means of the Claus process, there arises a problem that the sulfur recovery ratio is about 95% at most unless a tail gas treatment (TGT) is introduced because the acidic gas is accompanied by heavy hydrocarbons to a large extent. Additionally, there also arises a problem that soot is produced at a high rate in the sulfur recovery apparatus employing the Claus process.
In view of these problems, there has been proposed a technique of applying the Claus process not directly to the acidic gas separated by the physical absorption method in a sulfur recovery apparatus but after removing mercaptan, heavy hydrocarbons and carbon dioxide as off gas from the acidic gas. Then, the separated off gas is subjected to a hydrotreating process to convert mercaptan into hydrogen sulfide and the obtained hydrogen sulfide is subsequently processed by the Claus process (Jpn. Pat. Appln. Laid-Open Publication No. 9-255974. The sulfur recovery ratio is raised to 95 to 98% with this technique and further to about 99% when a TGT is introduced.
A technique of removing hydrogen sulfide and carbon dioxide by means of the chemical absorption method and subsequently using a molecular sieve to adsorb mercaptan that is not removed by the chemical absorption method is also known. With this technique, a dehydration tower is installed downstream relative to the acidic gas absorption apparatus that employs the amine process and a dehydration molecular sieve and a mercaptan adsorption molecular sieve are packed in the tower as upper and lower layers respectively (D. S. Clarke and P. W. Sibal, 77th Annual GPA Conversion, Mar. 16-18, 1998, Dallas Tex.). Since this technique does not use a physical absorption method for removing acidic gas from natural gas, heavy hydrocarbons are not absorbed and hence it is possible to raise the sulfur recovery ratio of the Claus process of recovering sulfur from the separated acidic gas. Additionally, it is possible with this technique to suppress the production of soot.
Furthermore, a technique of converting mercaptan in natural gas into hydrogen sulfide in advance and subsequently treating the natural gas by a chemical absorption method to remove hydrogen sulfide and carbon dioxide from the natural gas has also been developed. With this technique, mercaptan in natural gas is converted into hydrogen sulfide by injecting steam or water into natural gas to a ratio of 0.1 to 5 mol % relative to the natural gas under the condition of a temperature level of 260 to 350° C. and a pressure level of 4 to 10 MPa in the presence of a catalyst (Jpn. Pat. Appln. Laid-Open Publication No. 2002-265964). This technique provides advantages similar to those of the above described technique of adsorbing mercaptan by means of a molecular sieve because it does not involve the use of a physical absorption method.